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The conventional plastic packaging materials have been the major cause of pollution and depletion of resources in the environment. Biodegradable polymers from renewable resources can be considered as the most suitable for the solutions of these issues. This work is concerned with the development and analysis of new biodegradable polymers from algal biomass for packaging material. Chlorella vulgaris was employed as the feedstock for polymer synthesis and the process employed involved lipid and carbohydrate extraction and then polymerization. The synthesized polymers were analyzed by several techniques which include the FTIR spectroscopy, DSC, TGA, and SEM. The resultant polymers had good thermal stability with the decomposition temperatures starting from 250°C and above. Mechanical characterization gave tensile strength of the order of that of conventional plastics like polypropylene. The biodegradation studies carried out under solid waste composting conditions indicated that the entire material was degraded within 90 days. Thus, the research findings provide evidence to support the possibility of algal biomass to be used in the development of biodegradable packaging materials.

This research work is concerned with the development and analysis of new biodegradable polymers from algal biomass for packaging material. Chlorella vulgaris was employed as the feedstock for polymer synthesis and the process employed involved lipid and carbohydrate extraction and then polymerization.

Some works have been conducted in order to examine the prospective of algal biomass for biodegradable polymers production. Zeller, et al. (2013) cellulosed several polyurethanes from lipid derived from microalgae and proved their suitability for use in packaging. Gouveia et al. (2017) synthesized PHAs from the cyanobacteria, Spirulina sp. They also demonstrated that the obtained polymers possessed mechanical characteristics that could make them suitable for the packaging of food stuffs. Thus, the majority of such works has been done on one of the components of algal biomass (fats or carbohydrates) and the characteristics of the obtained polymers have not been described in detail.

The aim of this work was to develop new biodegradable polymers from the whole algal biomass for packagings. C. vulgaris was chosen as the feedstock because of its high lipid and carbohydrate content and easy cultivation (Kim et al., 2014). The polymers were prepared through a two method o extraction of lipids and carbohydrates then polymerization with a new catalyst system. The polymers’ chemical structure, thermal properties, mechanical properties and biodegradation were analyzed through several techniques.

1.1 Materials

The biomass of Chlorella Vulgaris was bought from a local algae farm located at Algae Co., New Delhi (Aldrich, USA) was the source of all chemicals employed in this study and all were used without any further purification.

1.2 Biomass Characterization

Lipid and carbohydrate content was quantified in the algal biomass following the methods laid down by Blight and Dyer (1959) and Dubois et al. (1956) respectively. The amount of protein was estimated by the method of Bradford (Bradford, 1976). The ash content was computed by heating the biomass in a muffle furnace at 550°C for four hours and all the experiments were done in triplicate.

1.3 Polymer Synthesis

The polymers were prepared by a two step process. In the first step, lipids and carbohydrates were extracted from the algal biomass using a solvent mixture of chloroform and methanol (At a biomass-to-solvent ratio of 1:10 (w/v) the experiments were prepared using 1 v/v of the solvent. The extraction was done at 60°C for 2 hours with stirring. The extract was obtained from the residual biomass through filtration and to remove the solvents, rotary evaporation was used. In the second step, the extracted lipids and carbohydrates were polymerized using a novel catalyst system consisting of tin(II) 2-ethylhexanoate and 1,5,7-triazabicyclo[4. 4. 0]dec-5-ene (TBD) at a molar ratio of 1:1. The polymerization was performed in a closed autoclave at 180°C, for 6 hours with the use of nitrogen gas. The obtained polymer was then purified through dissolving it in chloroform and then precipitating it in methanol. The polymer was dried under vacuum at 60°C for 24 h.

1.4 Polymer Characterization

The characterization of chemical structure of the polymers, Fourier transform infrared (FTIR) spectroscopy was used with Nicolet iS50, Thermo Scientific, Waltham, USA in the attenuated total reflectance (ATR) mode. The thermal behaviours of the blends were also evaluated by Differential Scanning Calorimetry (DSC) and Thermo Gravimetric Analysis (TGA) using Q2000 and Q500 models respectively from TA Instruments with the location at New Castle, DE, USA. For DSC, samples of the order of 10 mg were heated from -50 to 200°C at a rate of 10°C/min in nitrogen. For TGA, 5 mg of each sample was heated from 30 to 800°C at the heating rate of 10°C/min under nitrogen carrier gas.

The mechanical properties were assessed by applying tensile test in Instron 5943, tensile tester, USA following the ASTM D638 standard. Samples were prepared by compression molding at 180°C and cut into dog-bone shaped specimens with a gauge length of 25 mm and a width of 4 mm. The specimens were conditioned at 23°C and 50% relative humidity for 48 hours prior to testing. The testing was performed at a crosshead speed of 10 mm/min. At least five specimens were tested for each sample.

The morphology of the polymers was observed using scanning electron microscopy (SEM). Samples were cryofractured in liquid nitrogen, sputter-coated with platinum, and imaged at an accelerating voltage of 5 kV. The biodegradability of the polymers was assessed using a simulated composting test according to ISO 14855-1. Polymer films of approximately 100 mg were placed in a composting vessel containing a mixture of mature compost, sawdust, rabbit food, corn starch, sucrose, and urea. The vessel was incubated at 58°C and the evolution of CO₂ was measured over time using an infrared gas analyzer. The tests were conducted in triplicate and the results were expressed as percent biodegradation relative to the theoretical amount of CO₂ produced by complete mineralization of the sample.

1.1 Biomass Characterization

The proximate analysis of the C. Vulgaris biomass is given in Table 1. The lipids and carbohydrates components were established to be high at 42.5% and 30.2% respectively, a favorable feature for biodegradable polymer production. Protein and ash values were rather low at 20.1 and 7.2 for the respective product. The findings of this study are in agreement with other studies that have been carried out on the chemical makeup of C. Vulgaris biomass (Lee & Lim, 2015).

Table 1. Composition of C. Vulgaris biomass

1.2 Polymer Synthesis and Chemical Structure

The conversion of the lipids and carbohydrates into the polymer was 85% based on the mass obtained of the lipids and carbohydrates. The FTIR spectrum of the polymer is presented band at 3400 cm^-1 is the broad peak that is attributed to O-H stretching, which demonstrates existence of hydroxyl groups in the polymer. The bands observed at 2920 and 2850 cm-1 are assigned to the C-H stretching in the alkyl chain. The sharp peak at the 1740 cm-1 is assigned to the C=O stretching vibration of ester groups and the other peaks at 1160 and 1090 cm-1 are assigned to the C-O-C asymmetric and symmetric stretching of ether groups. From these results it can be concluded that the polymer is a polyester with both ester and ether linkages which is in agreement with the expected structure of the polymer derived from the polymerization of lipids and carbohydrates.

1.3 Thermal Properties

The polymer synthesized had a glass transition temperature of -5°C and a melting temperature of 120°C. The Tg of the polymer is comparatively low, which means that the polymer will be quite flexible at room temperature, which is a bonus for packaging applications. The melting peak is broad and has low enthalpy; this indicates that the polymer has low degree of crystallinity. This could be attributed to the fact that both lipid and carbohydrate monomers are incorporated into the polymer chain at random and thus the regular arrangement of the polymers is impaired.

The TGA thermogram of the polymer had a two-step decomposition curve with the initial decomposition temperature of 250°C and the second one of 400°C. The initial phase is the hydrolysis of ester group and the second phase is the hydrolysis of ether linkages and alkyl chain (Nguyen et al., 2015). Thus, the polymer had a residual mass of 5% at 800°C, which proved the thermal stability of the polymer.

1.4 Mechanical Properties

The mechanical properties of the polymer in tensile are given in Table 2. The polymer had a tensile strength of 25 Mpa, elongation at break of 10%, and a young modulas of 1.2 MPa. These values are in the range of other conventional plastics for instance polypropylene (PP) that has tensile strength of 30-40 Mpa and a young’s modulus of 1.1-1.5 GPa (Mark, 2009). The elongation at break of the polymer is reasonably low and this could be attributed to the low degree of crystallinity as seen earlier.

Table 2. Tensile properties of the polymer

1.5 Morphology

The SEM images of the cryofractured polymer used to determined fracture surface of the polymer and to be free of any discernible lipid and carbohydrate phase demarcation. This means that the polymer does not have any pores or voids in its structure and this is an advantage in packaging where the product should not allow gases or moisture to pass through.

1.6 Biodegradability

The polymer showed a lag phase of about 20 days in which no apparent biodegradation happened. This lag phase is often reported in biodegradation studies and it is postulated that it is the time taken by the microbial population to acclimatise on the polymer substrate (Shah et al., 2014). The biodegradation process started with the lag phase and then increased drastically; 60% biodegradation was observed within 50 days and 90% within 90 days. The polymer was fully disintegrated at the end of 120 days which shows that the polymer is highly biodegradable under composting environment.

The degradation of the polymer is fast because of the chemical bonds present in the polymer which are hydrolyzable ester bonds and thus they are easily broken down by microorganisms. Another reason to include hydrophilic carbohydrate units into the polymer chain is that the some of the enzyme can bind to these units and hydrolyse the polymer (Tokiwa & Calabia, 2007). The results of this study indicate that the polymer degrades quickly in composting conditions and thus, it does not pose a threat to the environment in industrial composting facilities.

These results show that algal biomass maybe a viable feedstock for the production of biodegradable polymers for packaging materials. The polymers have thermal, mechanical and biodegradation properties that qualify them to be used in packaging products such as films, containers and molded parts. The whole algal biomass, when used as the starting material, has many benefits over traditional plant feedstocks, these include; high productivity, short generation time, and the ability to grow on marginal or non-arable land and even on wastewater.

New biodegradable polymers were prepared from algae biomass and their properties were investigated for their suitability in utilizing green packaging materials. The polymers were prepared through a two step process in which lipids and carbohydrates were first extracted from the biomass and then polymerized using tin(II) 2-ethylhexanoate/ TBD catalyst system. Polymers synthesized had ester and ether linkage which were established by Fourier Transform Infrared Spectroscopy (FTIR). The thermal behavior of the polymers was analyzed with the help of DSC and TGA which showed the glass transition temperature of -5°C, melting temperature of 120°C and the decomposition temperature above 250°C. To analyze the mechanical characteristics, tensile testing was applied, which showed the tensile strength of 25 MPa, the elongation at break of 10%, and the Young’s modulus of 1.2 GPa that is close to the values of conventional plastics, for example, PP.

The surface morphology analysis of the polymers was done by SEM, which revealed that the polymers had a dense structure with no separate phases and no porosity. Biodegradation was evaluated by a simulated composting test, whereby the polymers were completely mineralized in 120 days, thus showing good biodegradation under composting conditions. The properties of the polymers regarding their environmental impact should be investigated in detail, with regard to biodegradation in various settings, for instance, in soil, water, and marine environments. The toxicity of any degradation products should also be analyzed to know the effect of the polymers to the environment and their management. This study proves that biodegradable polymers with high performance can be obtained from algal biomass and reveals the possibility of its usage in the sustainable packaging production. With further advancement and fine-tuning, the present algae-derived polymers have the potential to replace traditional plastics which will assist in the eradication of pollution and shift towards a circular economy. More studies should be conducted for better polymer synthesis and assess the effectiveness of the polymers in practical packaging scenarios. This involves comparing the various catalyst systems, reaction conditions and post polymerization treatments on the properties of the polymers. The cost analysis and possible ways of increasing the scale of the process should also be considered to evaluate its practicability in the market.

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